Zoom lens system

Abstract

A zoom lens system includes a positive first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group having a plurality of lens elements. During zooming from the short to long focal length extremities, at least the first, third and fifth lens groups move in an optical axis direction. A surface closest to the image side of the zoom lens system is a concave surface facing the image side. The following condition (1) is satisfied: 0.52<( m 345 t/m 345 w )/( ft/fw )<0.70  (1), wherein m345w and m345t designate, at the short and long focal length extremities, respectively, the combined lateral magnification of the third, fourth and fifth lens groups when focused on an object at infinity; and fw and ft designate the focal length of the zoom lens system at the short and long focal length extremities, respectively.

What is claimed is: 
     
       1. A zoom lens system comprising a positive first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group, in that order from the object side,
 wherein, during zooming from the short focal length extremity to the long focal length extremity, at least said first lens group, said third lens group and said fifth lens group move in an optical axis direction thereof, 
 wherein said fifth lens group includes a plurality of lens elements, 
 wherein a surface closest to the image side of said zoom lens system is a concave surface facing the image side, and 
 wherein the following condition (1) is satisfied:
   0.52<( m 345 t/m 345 w )/( ft/fw )<0.70  (1), wherein
 
 
 m345w designates the combined lateral magnification of said third lens group, said fourth lens group and said fifth lens group at the short focal length extremity when focused on an object at infinity, 
 m345t designates the combined lateral magnification of said third lens group, said fourth lens group and said fifth lens group at the long focal length extremity when focused on an object at infinity, 
 fw designates the focal length of said zoom lens system at the short focal length extremity, and 
 ft designates the focal length of said zoom lens system at the long focal length extremity. 
 
     
     
       2. The zoom lens system according to  claim 1 , wherein the following condition (2) is satisfied:
   0.4<( m 2 t/m 2 w )/( ft/fw )  (2), wherein
 
 m2w designates the lateral magnification of said second lens group at the short focal length extremity when focused on an object at infinity, 
 m2t designates the lateral magnification of said second lens group at the long focal length extremity when focused on an object at infinity, 
 fw designates the focal length of said zoom lens system at the short focal length extremity, and 
 ft designates the focal length of said zoom lens system at the long focal length extremity. 
 
     
     
       3. The zoom lens system according to  claim 1 , wherein said fourth lens group comprises a plurality of lens elements. 
     
     
       4. The zoom lens system according to  claim 1 , wherein said fifth lens group comprises a positive single lens element, and a cemented lens formed of a negative single lens element and a positive single lens element, in that order from the object side. 
     
     
       5. The zoom lens system according to  claim 4 , wherein the following condition (3) is satisfied:
   −2.0<( Rsi+Rdo )/( Rsi−Rdo )<−1.0  (3), wherein
 
 Rsi designates the radius of curvature of the surface on the image side of the positive single lens element that is provided closest to the object side within said fifth lens group, and 
 Rdo designates the radius of curvature of the surface on the object side of the cemented lens that is provided within said fifth lens group. 
 
     
     
       6. The zoom lens system according to  claim 4 , wherein the following condition (4) is satisfied:
   ( np−nn )/ Rb× 1000<−3.0  (4), wherein
 
 np designates the refractive index at the d-line of the positive single lens element of said cemented lens that is provided within said fifth lens group, 
 nn designates the refractive index at the d-line of the negative single lens element of said cemented lens that is provided within said fifth lens group, and 
 Rb designates the radius of curvature of a cemented surface of said cemented lens that is provided within said fifth lens group. 
 
     
     
       7. The zoom lens system according to  claim 1 , wherein a surface closest to the image side of said fifth lens group is a concave surface facing the image side. 
     
     
       8. A zoom lens system comprising a positive first lens group, a negative second lens group, a positive third lens group, a negative fourth lens group, and a positive fifth lens group, in that order from the object side,
 wherein, during zooming from the short focal length extremity to the long focal length extremity, at least said first lens group, said third lens group and said fifth lens group move in an optical axis direction thereof, 
 wherein a surface closest to the image side of said zoom lens system is a concave surface facing the image side, and 
 wherein the following conditions (1) and (2) are satisfied:
   0.52<( m 345 t/m 345 w )/( ft/fw )<0.70  (1), and
 
   0.4<( m 2 t/m 2 w )/( ft/fw )  (2), wherein
 
 
 m345w designates the combined lateral magnification of said third lens group, said fourth lens group and said fifth lens group at the short focal length extremity when focused on an object at infinity, 
 m345t designates the combined lateral magnification of said third lens group, said fourth lens group and said fifth lens group at the long focal length extremity when focused on an object at infinity, 
 m2w designates the lateral magnification of said second lens group at the short focal length extremity when focused on an object at infinity, 
 m2t designates the lateral magnification of said second lens group at the long focal length extremity when focused on an object at infinity, 
 fw designates the focal length of said zoom lens system at the short focal length extremity, and 
 ft designates the focal length of said zoom lens system at the long focal length extremity. 
 
     
     
       9. The zoom lens system according to  claim 8 , wherein said fourth lens group comprises a plurality of lens elements. 
     
     
       10. The zoom lens system according to  claim 8 , wherein said fifth lens group comprises a positive single lens element, and a cemented lens formed of a negative single lens element and a positive single lens element, in that order from the object side. 
     
     
       11. The zoom lens system according to  claim 10 , wherein the following condition (3) is satisfied:
   −2.0<( Rsi+Rdo )/( Rsi−Rdo )<−1.0  (3), wherein
 
 Rsi designates the radius of curvature of the surface on the image side of the positive single lens element that is provided closest to the object side within said fifth lens group, and 
 Rdo designates the radius of curvature of the surface on the object side of the cemented lens that is provided within said fifth lens group. 
 
     
     
       12. The zoom lens system according to  claim 10 , wherein the following condition (4) is satisfied:
   ( np−nn )/ Rb× 1000<−3.0  (4), wherein
 
 np designates the refractive index at the d-line of the positive single lens element of said cemented lens that is provided within said fifth lens group, 
 nn designates the refractive index at the d-line of the negative single lens element of said cemented lens that is provided within said fifth lens group, and 
 Rb designates the radius of curvature of a cemented surface of said cemented lens that is provided within said fifth lens group. 
 
     
     
       13. The zoom lens system according to  claim 8 , wherein a surface closest to the image side of said fifth lens group is a concave surface facing the image side.